The preparation of thermo-responsive palladium ...

Surface & Coatings Technology 204 (2010) 2130–2135

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t

The preparation of thermo-responsive palladium catalyst with high activity for electroless nickel deposition Wen-Ding Chen a, Yuh Sung b, Chang-Pin Chang c, Yann-Cheng Chen c, Ming-Der Ger c,⁎ a b c

Graduate School of Defense Science, Chung Cheng Institute of Technology, National Defense University, 335 Taiwan, ROC Chung Shan Institute of Science and Technology, 325 Taiwan, ROC Department of Applied Chemistry & Materials Science, Chung Cheng Institute of Technology, National Defense University, 335 Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 9 August 2009 Accepted in revised form 25 November 2009 Available online 4 December 2009 Keywords: Palladium Nanoparticle Catalyst Electroless deposition

a b s t r a c t In this study, a temperature-responsive palladium catalyst for electroless nickel deposition was prepared. The noble metal nanoparticles that was reduced and stabilized by styrene-N-isopropylacrylamide oligomer (St-co-NIPAAm) showed good dispersion and excellent stability in the aqueous solution without surfactant and reductant in the mixture. The catalytic activity of St-co-NIPAAm/Pd is much higher than that of conventional Sn/Pd colloids. It was found from our result that a nickel film with dramatically enhanced adhesion is formed on the PET surface without special pretreatment step, indicating that St-co-NIPAAm was used not only as the adsorption sites for palladium, but also as an adhesion-promoting layer for the electrolessly deposited nickel on the PET surface. The properties of St-co-NIPAAm/Pd nanocomposite and deposition rate of electroless nickel deposition (EN) were characterized by transmission electron microscopy (TEM), quartz crystal microbalance (QCM), Fourier transform infrared spectrometry (FT-IR), gel permeation chromatography (GPC) and UV–Vis spectra. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Electroless deposition is an important technique for metallization and has received more and more attention mainly due to its capability for metallization of non-conducting materials such as polymers and ceramics. Therefore, it is extensively applied in flexible electronics [1], printed circuit board (PCB) industry [2], ink-jet printing technique, etc. [3]. Electroless metallization has to be proceeded by an activity step that introduces the catalytic sites (palladium atoms and ions) onto the surface to be metallized. Since the deposition of metal begins on the adsorbed catalytic palladium clusters, the adhesion of catalyst on nonconductor surface is very important which leads to strong metal deposition. However, electroless plating of metal on polymer is often limited by the weak interfacial bond strength which is caused by structural incompatibility between the substrate and metallizing material [4]. On the other hand, micro-fabrication of electronic and mechanical structures is a typical time-consuming and expensive process because of the complicated optical lithography system. In addition, a large amount of waste and chemicals is produced in traditional patterning process, thus, the ink-jet printing process which can print the desired patterns has been considered as the desired future process for mass production of many industrial devices [5–7]. Recently, several papers mentioned that ink-jet printing techniques can be integrated with

⁎ Corresponding author. Fax: + 886 3 3892494. E-mail address: [email protected] (M.-D. Ger). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.11.038

electroless metal deposition to pattern metal on polymer surface [8– 10]. In our previous study, an organic solvent-based ink containing Pd nanoparticles reduced by styrene oligomers (St/Pd) was ink-jet printed onto a PET substrate, then, a Ni pattern was fabricated successfully by electroless Ni plating. A major challenge in applying the direct ink-jet nanoparticle process is to maintain good dispersion of nanoparticles in the ink. Owing to the hydrophobic characteristic of styrene oligomer, the Pd nanoparticles reduced by styrene oligomers showed good dispersion in an organic solvent without any surfactant and reductant in the mixture [10]. However, a different strategy should be adopted to develop a water-borne ink. For this purpose, Pd nanoparticles should be reduced and immobilized by a hydrophilic polymer to ensure good dispersion. But this will generally result in poor adhesion of the nanoparticles on the hydrophobic substrate. To solve this dilemma, a different approach that utilizes a thermoresponsive polymer was proposed in this study. Poly-N-isopropylacrylamide (PNIPAAm), an interesting thermoresponsive polymer, gained its popularity mainly because it has a fully reversible lower critical solution temperature (LCST) in water at around 305 K (32 °C) which is close to the physiological temperature. When heated above 305 K, the polymer becomes hydrophobic and precipitates out from the aqueous solution; below the LCST it becomes completely soluble and forms a clear solution because of the transition into hydrophilic state [11]. This temperature sensitive property has also been observed in N-isopropylacrylamide oligomer and in cooligomers based on N-isopropylacrylamide [12–14]. Therefore, Pd nanoparticles stabilized by PNIPAAm can be stably dispersed in

W.-D. Chen et al. / Surface & Coatings Technology 204 (2010) 2130–2135

aqueous solution below 305 K, and good adhesion between PNIPAAm and various hydrophobic substrates can be easily achieved at a temperature above the LCST [15–17]. Since the sulfate end group of styrene oligomers can act as the reducing agent and reduce Pd ions to Pd atoms directly without using any extra reducing agent [10] and it has also been reported that Pd can be directly chemisorbed on polymer surfaces carrying nitrogenated species because of the strong chemical affinity of palladium towards nitrogen [4], therefore, we have developed a water-borne ink based on St-co-NIPAAm/Pd in this study. The styrene-co-NIPAAm oligomer with sulfate group on the chain end was synthesized first by freeradical polymerization and then it was used to reduce and stabilize the Pd nanoparticles without any surfactant. Pd nanoparticles were further used as the activator for catalyzing electroless nickel (EN). The catalytic activity and stability of St-co-NIPAAm/Pd were compared with those of conventional Sn/Pd colloids. In addition, the effect of NIPAAm units on the improvement of adhesion between the nickel film and flexible PET substrate was also elucidated. 2. Experiments 2.1. Synthesis of styrene-co-NIPAAm oligomer All chemicals in this work were of analytic grade purity and acquired from Merck. Styrene was purified at reduced pressure before use. All other chemicals were used as received. The styrene-coNIPAAm oligomer was prepared by copolymerization of these two monomers in 50% acetone aqueous solution with potassium persulfate as the initiator. NIPAAm (0.5 mol) and styrene (0.25 mol) were mixed and then added into a 100 ml acetone aqueous solution of 3.4 × 10− 3 M potassium persulfate. The polymerization reaction was carried out at 343 K for 8 h. The as-synthesized co-oligomer was dialyzed against deionized water for 48 h in order to remove the unreacted monomers. The dialysis bath was changed six times during the course of the 48 h dialysis. The conversion of copolymerization is about 60 W%. The number average molecular weight and polydispersity index of styrene-co-NIPAAm oligomer, determined by GPC (WATERS GPC-150CV), is 1033 g mol− 1 and 1.71, respectively. Fourier transform infrared (FT-IR) spectra were taken using a BRUKER TENSOR 27 FT-IR spectrometer. The composition of the styrene-coNIPAAm oligomer determined from analysis of the 1H NMR (VARIAN UNITYINOVA 500 MHz NMR spectrometer) spectra of the co-oligomer dissolved in deuterated chloroform, is about 1/1. 2.2. Preparation of thermo-responsive Pd nanocomposite (Pd @ styrene-co-NIPAAm oligomer)

tix DMP-2811 printer onto a substrate at room temperature. The printed Pd was the catalyst for subsequent Ni–P deposition. The advantage of using thermal-responsive Pd as the activator for electroless nickel plating was examined in two ways: (1) Two electroless plating processes were adopted dependent on the activator used. The first process is based on the use of the commercial Pd/Sn colloids with a concentration of 600 ppm as the activator which follows a conventional electroless plating process involves cleaning, activating, accelerating and the final electroless plating. The second process involves a newly developed activation process which is carried out by immersing, respectively, the clean and untreated PET substrates into aqueous solutions of thermal-responsive Pd and sulfate terminated sodium polystyrene sulfonate/Pd (PSSNa/Pd) [10] with a concentration of 600 ppm for 5 min, followed by electroless plating. The composition of electroless plating bath for nickel deposition contains nickel sulfate (30 g/l), sodium hypophosphite (30 g/l) , sodium lactate (40 g/l), glycine (10 g/l), and ammonia solution (for pH adjustment). All these materials were prepared from SOWA pro-analysis grade chemicals. The electroless plating was carried out at 343 K for 5 min. The effect of different Pd activators on the adhesion between nickel deposit and PET was compared by simply bending the PET plates. (2) The activities of the Pd particles were determined by measuring the amount of deposition with quartz crystal microbalance (QCM). In the QCM experiment, the work electrode in the EN bath was prepared by dropping 3 μl of 600 ppm Pd nanoparticle solution uniformly on a 0.159 cm2 Au surface of the QCM substrate (SEIKO EG G QCA-917). The amount of nickel deposited could be measured by the frequency change of QCM. The QCM experiment was operated at 70 °C. 2.4. Contact angle measurement Contact angle measurement was used to demonstrate the hydrophilic/hydrophobic transition of the thermal-responsive Pd (poly(St-co-NIPAAm)/Pd). The Pd-nanoparticle catalyst layer was first coated on PET substrate. Then, the contact angle of electroless nickel aqueous solution (30 g/l) on the thermal-responsive Pd film was measured at two different temperatures using a face angle meter of the CA-D type which uses the sessile drop technique. 3. Results and discussion The FT-IR spectra of polystyrene, poly(N-isopropylacrylamide) and St-co-NIPAAm are shown in Fig. 1. The IR spectrum of pure poly (N-isopropylacrylamide) (Fig.1(a)) exhibited strong absorption peaks

After styrene-co-NIPAAm oligomer was synthesized, the styrene-coNIPAAm oligomer aqueous solution was poured into a 600 ppm PdCl2 aqueous solution of equal volume preheated to 348 K, and then the mixture was stirred for 10 min. After that, the color of the solution changed from yellow to black, which indicated the formation of Pd nanoparticles. The images of nanoparticles were obtained using a JEM-1230 transmission electron microscope. Samples for TEM were prepared by putting a few drops of the solution on formvar-carbon-film Cu grids (300 mesh, 3 mm). UV–Visible measurements were performed with a CARY 500 UV–Vis NIR spectrometer coupled with a temperature controller. The particle size of aggregated Pd nanoparticles was measured by a Zeta Plus (Brookhaven Instruments Co.) instrument. 2.3. Ink-jet patterning and electroless plating of nickel on a flexible PET substrate Pd nanoparticles (2 wt.%) suspended by water were printed by a commercially available piezoelectric Drop-on-Demand ink-jet Dima-

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Fig. 1. FT-IR spectra of polymers: (a) PNIPAAm; (b) PS; (c) St-co-NIPAAm.

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at 3247 cm− 1 (–NH of –CONH–), 3073 cm− 1 (CH of –CH–CH–), 2978 cm− 1, 2937 cm− 1 (–CH3 of –CH (CH3)2), and 2878 cm− 1 (CH of –CH (CH3)2), and the peaks at 1635 cm− 1 as well as 1580 cm− 1 are assigned to the typical amide I and amide II. The IR spectrum of PS (Fig.1(b)) shows the characteristic absorption band of aromatic C–H stretching at 3026 cm− 1, and bands of aliphatic C–H stretching at 2922 and 2852 cm− 1, respectively. Bands at 1453 and 1492 cm− 1 are related to the C–C skeletal inplane vibration. The out-of-plane deformation of C–H appears at 698 cm− 1. The peak at 1375 cm− 1 is identified as the asymmetric stretching of the S–O bond. The symmetric vibration of this S–O bond produces the characteristic split bands at 1150–1185 cm− 1 [18]. The IR spectrum of St-coNIPAAm (Fig.1(c)) revealed well-defined bands at about 697, 757, and 1493 cm− 1, a characteristic of the benzene ring of polystyrene, and peaks at 1635, 1580 cm− 1 (C O of –CONH–) as well as at 3247 cm− 1 (–NH of –CONH–), a characteristic of NIPAAm [19]. It indicates that the copolymer of styrene and PNIAAm is formed. It was reported that the specific activity Q and polarity e for Nisopropylacrylamide are Q = 0.26 and e = −0.71 [20]. The monomer reactivity ratios for N-isopropylacrylamide-styrene monomer pair were calculated by using the Palfrey–Price scheme. The calculated values are rNIPAAm = 0.112 and rst = 3.87, indicating that styrene has a higher reactivity than that of NIPAAm. The product of rNIPAAm·rst is less than 1, showing that the copolymers should be mainly random. Since rst N 1 and rNIPAAm ≪ 1, there is a tendency toward formation of long styrene segments at the beginning of the polymerization and the styrene segments are separated by some NIPAAm units. When the NIPAAm monomer is much more than styrene in the feed (two fold in our case), relatively long NIPAAm chain, which is also separated by styrene units, is synthesized at the end of the reaction interval. Thus, the resultant copolymer can be depicted as a “diblock” copolymer. It provides an extra advantage. The LCST of PNIPAAm can be changed by incorporation of comonomer of the PNIPAAm backbone. However, it has been reported that the comonomer content cannot be increased much in the case of statistical copolymer [21]. It might lead to the disappearance of LCST as the copolymers turn water soluble or water insoluble at every temperature. The co-oligomer synthesized in this method, thus, can provide us a plausible way to adjust the LCST by simply changing the feed ratio of monomers. The variation of transmittance with temperature for the oligomer used in this study is displayed in Fig. 2. The transition is rather broad, which might be attributed to the chemical heterogeneity of the copolymers. The turbidity of St-co-NIPAAm aqueous solution changed gradually with the increase of temperature but increased deeply after 305 K, which might be considered as the LCST of St-co-NIPAAm used.

Our previous results have shown that polystyrene including PS microsphere, PS oligomer and PS-PMMA co-oligomer with the presence of end-capped sulfate groups can be utilized to reduce gold nanoparticles without adding extra reducing agent into the mixture [22,23]. It strongly suggested that the presence of end functional S–O bond is vital in the preparation of metal nanoparticles by this method. A possible reduction mechanism has been proposed and published elsewhere [24]. In this study, potassium persulfate was used as the initiator so that the oligomer chains were capped with sulfate groups. The sulfate group could reduce Pd2+ to form Pd nanoparticles on the surface of this thermo-responsive oligomer. The formation of St-co-NIPAAm/Pd nanocomposite is confirmed by TEM image. Fig. 3 compared the TEM image of synthesized St-co-NIPAAm/ Pd nanocomposites (Fig. 3a) with that of commercial Pd/Sn colloids (Fig. 3b). Fig. 3(a) displays the TEM image of nanoparticles which were prepared as described in the Experiments section. It indicates that self-assembled Pd nanoparticles were successfully reduced on the surface of the thermal-sensitive co-oligomer without adding any reducing agent. It is seen from Fig. 2 that the diameters of Pd/Sn colloid and St-co-NIPAAm/Pd nanocomposite are 5–150 and 3–5 nm, respectively. The comparison between these TEM images revealed that Pd nanoparticles reduced by St-co-NIPAAm shows not only smaller diameters but also better dispersion than those of Pd/Sn colloids. The results can be best understood by the fact that the metal nuclei reduced by the oligomers adsorbed and grew on specific sites of oligomer surfaces, which prevents Pd particles from self aggregation. It is noteworthy that, as these oligomers act as stabilizers and well prevent colloids from aggregation, the as-prepared colloids were very stable at room temperature in the solvent. Generally, in electroless metal deposition, Pd/Sn colloids act as catalyst in the transfer of electrons from reducing agent to metal ions. Therefore, the catalytic oxidation power of reducing agent in reducing metal ions depends on the catalyst surface-area-to-volume ratio. Since St-co-NIPAAm/Pd nanoparticles have a smaller and more uniform size than Pd/Sn colloid

Fig. 2. Variation of light transmittance with temperature.

Fig. 3. TEM images of Pd nanoparticles: (a) St-co-NIPAAm/Pd nanoparticles (b) Sn/Pd.


particles, it is expected that St-co-NIPAAm/Pd will have a higher catalytic activity than that of conventional Pd/Sn colloid. The UV–Visible spectra were measured for poly(St-NIPAAm)/Pd solutions with different standing time at room temperature. It is obvious from Fig. 4(a) for as-prepared Pd nanocomposites that there is no peak appearing at 325 nm and 440 nm, which are the characteristic peaks of Pd(II) ion [25]. It has been reported that the Pd nanoparticles show only a broad absorption peak within the UV–Visible region due to d–d interband transitions [26]. Therefore, the absence of absorption peaks within UV–Visible region for the as-prepared Pd nanocomposites can be ascribed to the complete reduction of Pd(II) ions to Pd0 by St-coNIPAAm oligomers. The UV–Visible spectra remained virtually unchanged up to 96 h (Fig. 4(b) to (d)). This result suggests the stability of St-co-NIPAAm/Pd nanoparticles. The images of Pd solutions after standing for 96 h at room temperature are shown in Fig.5. Fig. 5(a) shows that the color of St-coNIPAAm/Pd solution is black and very uniform as in the initial solution. In Fig. 5(b), however, Pd/Sn colloids clearly show the existence of two layers. The top layer is brown and the bottom one is dark-brown. It is clear that Sn2+ is oxidized into Sn4+, resulting in the reduction of Pd2+ to Pd0 and the aggregation [25]. The average size of the particles is 35 nm for St-co-NIPAAm/Pd and 3.3 μm for Pd/Sn, respectively. The different behaviors mainly come from the different stabilities of catalytic nanoparticles. It is clear from Figs. 4 and 5 that the stability of St-co-NIPAAm/Pd is much better than that of Pd/Sn in both spectral response and physical appearance. The Pd catalytic activity was evaluated by measuring in situ the EN deposition rate using QCM. The deposition rates are recalculated from the electrode frequency changes according to the Sauerbrey's equation [27]. The frequency changes and calculated deposition rates are summarized in Table 1. The average deposition rates at 180 s are about 1491.5 and 615.7 μg/cm2 s for St-co-NIPAAm/Pd and Pd/Sn, respectively. The deposition rates at 300 s increase to about 1997.3 and 1005.8 μg/cm2 s for St-co-NIPAAm/Pd and Pd/Sn, respectively. Therefore, it is evident that the synthesized St-co-NIPAAm/Pd nanoparticles show higher activity than Pd/Sn as the activator for EN plating. This can be ascribed to the smaller size and more uniform dispersion of St-co-NIPAAm/Pd than those of Pd/Sn. In our previous article, the reported deposition rates for St/Pd and Pd/Sn catalyst systems were 5954.7 and 4160.3 ng/cm2 s, respectively [10].The catalytic activities of St-co-NIPAAm/Pd and St/Pd cannot be compared directly since the experimental conditions such as temperature and concentration of activator solution are not the same. However, the ratio of the deposition rate of St-co-NIPAAm/Pd system to that of Sn/ Pd in current study is 1.98 (300 s), while that for St/Pd and Pd/Sn in the previous study is 1.43. The conclusion can be drawn from these two ratios that the catalytic activity of St-co-NIPAAm/Pd is higher than that of St/Pd. A preliminary explanation for this phenomenon is as follows: when styrene oligomers were applied, only Pd(0) particles

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Fig. 5. The images of colloids after standing for 96 h: (a) St-co-NIPAAm/Pd; (b) Sn/Pd colloids.

reduced by the sulfate end groups were adsorbed on the oligomer surface. However, with NIPAAm units existing on the backbone of the oligomer, in addition to the Pd(0) adsorbed on the end group sites, extra Pd(+ 2) ions can be directly chemisorbed on polymer surfaces via formation of coordination bonds between the metal ions and amide groups of NIPAAm. Since the electroless plating was operated at a temperature of 343 K, which is far above LCST of the co-oligomer, the conformation of adsorbed St-co-NIPAAm oligomer layer on PET substrate is, thus, compact. It is reasonable to consider that the surrounding NIPAAm around Pd might partly cover the active sites of Pd nanoparticles and decrease the catalytic activity. However, it is clear from deposition rate results that St-co-NIPAAm/Pd still has better catalytic properties than that of St/Pd. Fig. 6 displays the contact angles of catalytic poly(St-co-NIPAAm)/ Pd film at 298 K and 323 K, respectively. As the temperature increased from 298 K to 323 K, the contact angle of electroless nickel solution on the thermal-sensitive catalytic film increased from 7° to 48°, which could result from the phase behavior of NIPAAm segments. The obvious change of contact angle indicated that the wettability changed in the catalytic poly(St-co-NIPAAm)/Pd film which went through a transition from hydrophilic at lower temperature to hydrophobic at higher temperature owing to the LCST effect of Stco-NIPAAm oligomer. The water contact angle on the bare PET surface is ca. 73° at room temperature [17], which is quite different from the value of 7° observed for the poly(St-co-NIPAAm) coated surface. It provided additional evidence that the PET surface is well covered with poly(St-co-NIPAAm) chains. In addition, Miura et al. [17] studied the temperature effect on the adsorption/desorption behavior of PNiPAAm on various substrates and concluded that the interaction between PNiPAAm and most hydrophobic polymer surfaces is mainly due to hydrophobic interactions and most of the adsorbed polymer is easily rinsed off at rinse temperatures below the LCST. Unlike their results for adsorbed NIPAAm, however, our result shows that the adsorbed poly(St-co-NIPAAm)/Pd film remains strongly bound to a substrate at rinse temperature below LCST. It indicates the interaction between styrene unit and PET is crucial in the adhesion strength

Table 1 Summary of kinetic parameters of electroless nickel deposition analyzed by QCM. Pd catalyst (600 ppm)

Fig. 4. UV–Vis spectra of the St-co-NIPAAm/Pd solutions with different standing time. (a) 0 h; (b) 0.5 h; (c) 1.5 h; (d) 96 h.

(A) Induction time (B) Frequency change (C) Deposition rate (s) (Hz) (μg/cm2 s)

St-co-NIPAAm/Pd 180 300 Sn/Pd 180 300

− 290,500 − 389,000 − 206,900 − 338,000

1491.5 1997.3 615.7 1005.8

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Fig. 6. The contact angle of electroless nickel solution on the thermal-sensitive catalytic film St-co-NIPAAm/Pd at different temperatures (a) 25 °C; (b) 50 °C.

which influences the adsorption and retention of the St-co-NiPAAm/ Pd catalysts. After electroless nickel plating, the adhesion between the deposited layer and PET substrate was simply examined by bending the substrate with hand. Since the surface activity of the polymer substrate with metals is generally low, the adhesion between the polymer surface and metallic layer is commonly enhanced by chemical etching. Without the etching step, the adhesion between PET and the nickel plated film deposited with Pd/Sn as both the sensitizer and activator is poor as expected, as shown in Fig. 7(b). The deposited layer is detached completely from PET substrate after bending. Fig. 7(c) shows that the adhesion between PET and the nickel plated film deposited with PSSNa/

Pd as the activator is still poor owing to the highly hydrophilic nature of PSSNa/Pd. However, a totally different result was observed in Fig. 7(a), in which St-co-NIPAAm/Pd was used. The nickel film was not detached from the PET substrate after bending. It indicates that the strong adhesion of the thermal-sensitive oligomer to PET is ensured without special pretreatment. A mechanism is, thus, proposed to explain this result. The schematic plots of this proposed mechanism are displayed on Fig. 8. During the inkjet printing stage, it is reasonable to consider that St-co-NIPAAm molecules are anchored to the surface at the styrene units along the oligomer backbone owing to the existence of the hydrophobic interaction between styrene units and PET, while the hydrophilic

Fig. 7. The EN pattern fabricated by Pd catalyst on PET. (a) St-co-NIPAAm/Pd; (b) Sn/Pd; (c) PSSNa/Pd.

Fig. 8. Schematic illustration of the phase transition of thermo-responsive catalyst during electroless deposition process: (a) activation; (b) heating during electroless nickel deposition; (c) metallization.


NIPAAm chain segments might extend into the solution since the temperature of printed ink is lower than the LCST of NIPAAm (Fig. 8(a)). NIPAAm chain segments dehydrate and become hydrophobic as EN starts at a temperature of 343 K, resulting in the fixing of NIPAAm chain segments on the PET surface (Fig. 8(b)). It improves the interfacial adhesion significantly, and a nickel film with dramatically enhanced adhesion is formed on PET surface. 4. Conclusion Radical copolymerization of N-isopropylacrylamide (NIPAAm) with styrene (St) was carried out in the presence of potassium persulfate (KPS) as an initiator at 70 °C. The oligomer (St-co-NIPAAm) synthesized with the feed monomer ratio of styrene-N-isopropylacrylamide equal to 1/2 was associated to form random copolymer with a tendency toward formation of long styrene segments at the beginning of the polymerization and the styrene segments are separated by some NIPAAm units. Relatively long NIPAAm chain, which is also separated by styrene units, is synthesized at the end of the reaction interval. The resulting oligomer can be used to reduce Pd ions into Pd nanoparticles without adding any extra reducing agent. The St-co-NIPAAm-stabilized palladium nanoparticles are well dispersed in aqueous solution and can be used as an activator for electroless nickel deposition. The deposition rate of St-coNIPAAm/Pd nanoparticle system was higher than that of conventional Sn/Pd colloids, indicating that St-co-NIPAAm/Pd is more active than conventional Pd/Sn colloid as an activator. St-co-NIPAAm was used not only as absorption sites for the palladium, but also as an adhesionpromoting layer for the electrolessly deposited nickel on the PET surface. References [1] T.F. Guo, S.C. Chang, S. Pyo, Y. Yang, Langmuir 18 (2002) 8142. [2] S. Siau, A. Vervaet, L. Degrendele, Johan De Baets, A.V. Calster, Appl. Surf. Sci. 252 (2006) 2717.

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